The present invention relates generally to the field of anticounterfeiting and authentication methods and devices and, more particularly, to methods, security devices and apparatuses for authentication of documents and valuable articles using the intensity profile of moire patterns.
Counterfeiting of documents such as banknotes is becoming now more than ever a serious problem, due to the availability of high-quality and low-priced color photocopiers and desk-top publishing systems (see, for example, xe2x80x9cMaking Moneyxe2x80x9d, by Gary Stix, Scientific American, March 1994, pp. 81-83). The same is also true for other valuable products such as CDs, DVDs, software packages, medical drugs, etc., that are often marketed in easy to falsify packages.
The present invention is concerned with providing a novel security element and authentication means offering enhanced security for banknotes, checks, credit cards, identity cards, travel documents, industrial packages or any other valuable articles, thus making them much more difficult to counterfeit.
Various sophisticated means have been introduced in prior art for counterfeit prevention and for authentication of documents or valuable articles. Some of these means are clearly visible to the naked eye and are intended for the general public, while other means are hidden and only detectable by the competent authorities, or by automatic devices. Some of the already used anti-counterfeit and authentication means include the use of special paper, special inks, watermarks, micro-letters, security threads, holograms, etc. Nevertheless, there is still an urgent need to introduce further security elements, which do not considerably increase the cost of the produced documents or goods.
Moire effects have already been used in prior art for the authentication of documents. For example, United Kingdom Pat. No. 1,138,011 (Canadian Bank Note Company) discloses a method which relates to printing on the original document special elements which, when counterfeited by means of halftone reproduction, show a moire pattern of high contrast. Similar methods are also applied to the prevention of digital photocopying or digital scanning of documents (for example, U.S. Pat. No. 5,018,767 (Wicker), or U.K. Pat. Application No. 2,224,240 A (Kenrick and Jefferson)). In all these cases, the presence of moire patterns indicates that the document in question is counterfeit. Other prior art methods, on the contrary, take advantage of the intentional generation of a moire pattern whose existence, and whose precise shape, are used as a means of authenticating the document. One known method in which a moire effect is used to make visible an image encoded on the document (as described, for example, in the section xe2x80x9cBackgroundxe2x80x9d of U.S. Pat. No. 5,396,559 (McGrew)) is based on the physical presence of that image on the document as a latent image, using the technique known as xe2x80x9cphase modulationxe2x80x9d. In this technique, a uniform line grating or a uniform random screen of dots is printed on the document, but within the pre-defined borders of the latent image on the document the same line grating (or respectively, the same random dot-screen) is printed in a different phase, or possibly in a different orientation. For a layman, the latent image thus printed on the document is hard to distinguish from its background; but when a reference transparency comprising an identical, but unmodulated, line grating (respectively, random dot-screen) is superposed on the document, thereby generating a moire effect, the latent image pre-designed on the document becomes clearly visible, since within its pre-defined borders the moire effect appears in a different phase than in the background. However, this previously known method has the major flaw of being simple to simulate, since the form of the latent image is physically present on the document and only filled by a different texture. The existence of such a latent image on the document will not escape the eye of a skilled person, and moreover, its imitation by filling the form by a texture of lines (or dots) in an inversed (or different) phase can easily be carried out by anyone skilled in the graphics arts.
Other moire based methods, in which the presence of moire intensity profiles indicates the authenticity of the document, have been disclosed by the present inventors in U.S. Pat. No. 6,249,588 and its continuation-in-part U.S. Pat. No. 5,995,638. These methods completely differ from the above mentioned technique, since no phase modulation is used, and furthermore, no latent image is present on the document. On the contrary, all the spatial information which is made visible by the moire intensity profiles according to the inventions of the present inventors is encoded in the specially designed forms of the individual dots which constitute the dot-screens. These inventions are based on specially designed periodic structures, such as dot-screens (including variable intensity dot-screens such as those used in real, full gray level or color halftoned images), pinhole-screens, or microlens arrays, which generate in their superposition periodic moire intensity profiles of any chosen colors and shapes (letters, digits, the country emblem, etc.) whose size, location and orientation gradually vary as the superposed layers are rotated or shifted on top of each other. In U.S. Pat. No. 5,712,731 (Drinkwater et al.) another moire based method is disclosed which, unlike the above mentioned inventions, can be combined within a hologram or a kinegram, or with parallax effects due to the varying view angles of the observer. However, this last disclosure has the disadvantage of being limited only to the case where the superposed revealing structure is a microlens array and the periodic structure on the document is a constant dot-screen with identical dot-shapes throughout. Thus, in contrast to the present authors"" inventions, this disclosure excludes the use of dot-screens or pinhole-screens as revealing structures, as well as the use on the document of full, real halftoned images with varying tone levels (such as portraits, landscapes, etc.), either in full gray levels or in color, that are made of halftone dots of varying sizes and shapesxe2x80x94which are the core of the methods disclosed by the present inventors, and which make them so difficult to falsify.
In the present invention the present inventors disclose new methods largely improving their previously disclosed methods mentioned above, which make them even more difficult to counterfeit. These new improvements make use of the theory developed in the paper xe2x80x9cFourier-based analysis and synthesis of moires in the superposition of geometrically transformed periodic structuresxe2x80x9d by I. Amidror and R. D. Hersch, Journal of the Optical Society of America A, Vol. 15, 1998, pp. 1100-1113 (hereinafter, xe2x80x9c[Amidror98]xe2x80x9d), and in the book xe2x80x9cThe Theory of the Moire Phenomenonxe2x80x9d by I. Amidror, Kluwer, 2000 (hereinafter, xe2x80x9c[Amidror00]xe2x80x9d). According to this theory it is possible, by using certain mathematical rules that will be explained in detail below, to synthesize aperiodic, geometrically transformed structures which in spite of being aperiodic in themselves, still generate, when they are superposed on top of one another, periodic moire intensity profiles with clearly visible and undistorted elements, just like in the periodic cases disclosed by the present inventors in their previous U.S. Pat. No. 6,249,588 and its continuation-in-part U.S. Pat. No. 5,995,638. Furthermore, it will be disclosed here how even cases which do not yield periodic moires can still be advantageously used for anticounterfeiting and authentication of documents and valuable articles in accordance with the present invention.
It should be noted that the approach on which the present invention is based further differs from that of prior art in that it not only provides full mastering of the qualitative geometric properties of the generated moire (such as its geometric layout), but it also enables the intensity levels of the generated moire to be quantitatively determined.
The present invention relates to new methods, security devices and apparatuses for authenticating documents (such as banknotes, trust papers, securities, identification cards, passports, etc.) or other valuable articles (such as optical disks, CDs, DVDs, software packages, medical products, etc.). In order to fully understand the present invention and its advantages, it would be useful to summarize first the principles of the original methods disclosed by the present inventors in U.S. Pat. No. 6,249,588 and its continuation-in-part U.S. Pat. No. 5,995,638. These methods are based on the moire intensity profiles which are generated between two or more specially designed periodic dot-screens, at least one of which being located on the document itself. Each periodic dot-screen consists of a lattice of tiny dots, and is characterized by three parameters: its repetition frequency, its orientation, and its dot shapes. These periodic dot-screens are similar to dot-screens which are used in classical halftoning, but they have specially designed dot shapes, frequencies and orientations. When the second dot-screen (or a corresponding microlens array) is laid on top of the first dot-screen, in the case where both of them have been designed in accordance with the inventors"" disclosures, there appears in the superposition a highly visible repetitive moire pattern of a predefined intensity profile shape, whose size, location and orientation gradually vary as the superposed layers are rotated or shifted on top of each other. As an example, this repetitive moire pattern may comprise any predefined letters, digits or any other preferred symbols (such as the country emblem, the currency, etc.).
In the present invention, the same inventors disclose new methods which are even more difficult to counterfeit. According to the theory developed in [Amidror98] and [Amidror00] it is possible, by using certain mathematical rules that will be explained in detail below, to synthesize aperiodic, geometrically transformed structures which in spite of being aperiodic in themselves, still generate, when they are superposed on top of one another, periodic moire intensity profiles with clearly visible and undistorted elements, just like in the periodic cases disclosed by the present inventors in their previous U.S. Pat. No. 6,249,588 and its continuation-in-part U.S. Pat. No. 5,995,638. Furthermore, it is shown in the present disclosure how even cases which do not yield periodic moires can still be advantageously used for anticounterfeiting and authentication of documents and valuable articles. In all of these new cases, each dot-screen is also characterized by a fourth parameter, in addition to the three parameters that were already mentioned above in the periodic case. This fourth parameter is the geometric transformation which has been applied to the originally periodic dot-screen in order to obtain the aperiodic, geometric transformed dot-screen in accordance with the present disclosure.
When the second dot-screen (hereinafter: xe2x80x9cthe master screenxe2x80x9d) is laid on top of the first dot-screen (hereinafter: xe2x80x9cthe basic screenxe2x80x9d), in the case where both screens have been designed in accordance with the present disclosure, there appears in the superposition a highly visible repetitive moire pattern of a predefined intensity profile shape. For example, the repetitive moire pattern may consist of any predefined letters, digits or any other preferred symbols (such as the country emblem, the currency, etc.).
As disclosed in U.S. Pat. No. 5,275,870 (Halope et al.) it may be advantageous in the manufacture of long lasting documents or documents which must withstand highly adverse handling to replace paper by synthetic material. Transparent sheets of synthetic materials have been successfully introduced for printing banknotes (for example, Australian banknotes).
The present invention concerns new methods for authenticating documents which may be printed on various supports, including (but not limited to) such transparent synthetic materials. It should be noted that the term xe2x80x9cdocumentsxe2x80x9d refers throughout the present disclosure to all possible printed articles, including (but not limited to) banknotes, passports, identity cards, credit cards, labels, optical disks, CDs, DVDs, packages of medical drugs or of any other commercial products, etc. Although the present invention may have several embodiments and variants, three embodiments of particular interest are given here by the way of example, without limiting the scope of the invention to these particular embodiments. In one embodiment of the present invention, the moire intensity profile shapes can be visualized by superposing a basic screen and a master screen which are both located on two different areas of the same document. In a second embodiment of the present invention, only the basic screen appears on the document itself, and the master screen is superposed on it by the human operator or the apparatus which visually or optically validates the authenticity of the document. In a third embodiment of this invention, the master screen is a sheet of microlenses (hereinafter: xe2x80x9cmicrolens structurexe2x80x9d). An advantage of this third embodiment is that it applies equally well to both transparent support, where the moire is observed by transmittance, and to opaque support, where the moire is observed by reflection. (The term xe2x80x9copaque supportxe2x80x9d as employed in the present disclosure also includes the case of transparent materials which have been made opaque by an inking process or by a photographic or any other process.)
The fact that moire effects generated between superposed dot-screens are very sensitive to any microscopic variations in the screened layers makes any document protected according to the present invention practically impossible to counterfeit, and serves as a means to distinguish easily between a real document and a falsified one.
It should be noted that the dot-screens which appear on the document itself in accordance with the present invention may be printed on the document like any screened (halftoned) image, within the standard printing process, and therefore no additional cost is incurred in the document production.
Furthermore, the dot-screens printed on the document in accordance with the present invention need not be of a constant intensity level. On the contrary, they may include dots of gradually varying sizes and shapes, and they can be incorporated (or dissimulated) within any variable intensity halftoned image on the document (such as a portrait, landscape, or any decorative motif, which may be different from the motif generated by the moire effect in the superposition). To reflect this fact, the terms xe2x80x9cbasic screenxe2x80x9d and xe2x80x9cmaster screenxe2x80x9d used hereinafter will also include cases where the basic screens (respectively: the master screens) are not constant and represent halftoned images. As is well known in the art, the dot sizes in halftoned images determine the intensity levels in the image: larger dots give darker intensity levels, while smaller dots give brighter intensity levels.
In the present disclosure different variants of the invention are described, some of which are intended to be used by the general public (hereinafter: xe2x80x9covertxe2x80x9d features), while other variants can only be detected by the competent authorities or by automatic devices (hereinafter: xe2x80x9ccovertxe2x80x9d features). In the latter case, the information carried by the basic screen is masked using any of a variety of techniques, as described by the present inventors in U.S. Pat. No. 5,995,638. The terms xe2x80x9cbasic screenxe2x80x9d and xe2x80x9cmaster screenxe2x80x9d as employed in the present disclosure include, therefore, both overt and covert cases.
Also described in the present disclosure is the multichromatic case, in which the dot-screens used are multichromatic, thereby generating a multichromatic moire effect.
The terms xe2x80x9cprintxe2x80x9d and xe2x80x9cprintingxe2x80x9d refer throughout the present disclosure to any process for transferring an image onto a support, including by means of a lithographic, photolithographic, photographic, electrophotographic or any other process (for example: engraving, etching, perforating, embossing, ink jet, dye sublimation, etc.).
The disclosures [Amidror98], [Amidror00], U.S. patent application Ser. No. 08/410,767 filed Mar. 27, 1995 (Ostromoukhov, Hersch), now U.S. Pat. No. 6,198,545, granted Mar. 6, 2001, and U.S. patent application Ser. No. 09/477,544 filed Jan. 4, 2000 (Ostromoukhov, Hersch) have certain information and content which may relate to the present invention and aid in understanding thereof.